The Mars Volta,, Astoria, London

The Mars Volta raise questions I'm not used to facing. Questions such as "Is it OK to like...?" Questions which, as someone who was born five years too late to feel the full fire-and-brimstone "you're either with us or against us" effect of punk, I never normally even consider.

The Mars Volta raise questions I'm not used to facing. Questions such as "Is it OK to like...?" Questions which, as someone who was born five years too late to feel the full fire-and-brimstone "you're either with us or against us" effect of punk, I never normally even consider.

But tonight, as I stand amid 1,500 people who are warmly applauding five solid minutes of fretboard noodling (while the rest of the band take their leave), previously unheard panic alarms start sounding in my brain.

The Mars Volta are the band formed by Omar Rodriguez and the wondrously-named Cedric Bixler Zavala after the demise of At The Drive-In, whose break-up, if TMV's radically different direction is anything to go by, genuinely can be ascribed to the trusty cliché "musical differences".

ATD-I were a taut, wiry, minimal, relatively straightforward garage rock proposition, a latterday MC5. The Mars Volta, by contrast, are making the most extreme, challenging, ambitious music of the modern age. There are those who will tell you that TMV have invented a whole new kind of music, but there are also those who will scoff that they've merely resurrected a whole old one. It's a sound whose complexity defies definition, but in three crass, simple words, it's jazz-punk-prog. And it's the second p-word which is the most problematic. Progressive rock still carries all kinds of taboo connotations. Concept albums, for example. And indeed, De-Loused In The Comatorium, The Mars Volta's debut, is just such a beast, written from the point of view of their friend who fell into a drug-induced coma before dying, and the adventures they imagine he might have had in his dreams.

They look the part, too. With their skinny hips squeezed into boot-cut cords, and their huge microphone-like Jewfro heads protruding from tight, open-necked shirts, they look like Nazareth or Ten Years After. (Actually, I have no idea what those bands looked like, but I'm taking a wild guess).

And boy, do they sound the part. This is a band who will surely find it impossible to replace a member: songs this multi-layered and episodic can surely only be learned by people who were there when they first took shape. This is literally spastic music (in the sense of sudden, uncontrolled movements). I keep thinking of Led Zeppelin at their most spatial: imagine that bit in "Whole Lotta Love" when it all goes haywire ("way down inside...") before the main riff kicks back in, then stretch that out over 20 minutes.

With their staggeringly lengthy songs and trademark false endings, The Mars Volta make a fool of the early clapper. The first time I saw them play, at this year's Reading Festival, I snuck off during the first song to catch a bit of Beck. When I came back, they were still playing the same song. A typical festival set might contain three songs. A full-length gig might contain seven. Tonight, in a hundred-minute set, I think I counted six, although it might have been five. As they say in America, you do the math.

It is, by turns, intensely exciting and excruciatingly dull (not many people here will admit to each other, or themselves, that they're bored, but it's noticeable that there are far fewer people here at the end than at the start).

Singer Cedric does everything humanly possible to liven things up, hurling his body dangerously around the stage, crashing his microphone onto the cymbals, James Brown strutting, breakdancing, speaker-diving, backflipping, cartwheeling, the works. It's difficult to hear what he's getting so worked up about - his bizarre helium voice makes all but a handful of words (the repeated "exoskeletal") inaudible, and his one speech to the audience appears to be something about the 40th anniversary of Dr Who (or was it JFK?) - but whatever it is, he's sure worked up about it.

Guitarist Rodriguez is almost as watchable. At one point he slings his guitar around his back, catches it again at the front, and carries on playing his quicksilver-fingertipped riff without missing a beat. Even the bassist has a solo spot, coaxing some utterly bizarre noises from his four strings while his bandmates just stand in a circle, arms folded, and watch.

The jury's still out on The Mars Volta's prog rock excesses, but they perform with enough berserk adrenalin to carry it off. The music that will be made by the inevitable Mars Volta imitators, however, doesn't bear thinking about.